January Jones didn't appear in the double bill opener of season five of Mad Men, but we mentioned in last week's review that her character Betty Francis cast a hefty shadow over proceedings. Now we know why that shadow was so significant.

Betty got fat. OK, so in the real world she would only be considered a bit plump and chubby. But in the land of TV and considering Jones's usual physique, she has piled on the pounds. And stuck in a grim old house with dull old husband Henry 24 hours a day, who can blame her if she wants to stuff her face with Cheetos and eat all her children's leftover ice cream sundaes?

Matthew Weiner may have had his hands forced into this Betty storyline because of Jones's real-life pregnancy, but the downbeat, frumpy and lifeless new Betty doesn't feel like a shoe-horned twist or unnecessary add-on.

Don and Betty will always have a strong Mad Men bond and her battle with the bulge and a throat cancer scare were enough to tangle them up together once again. Whether it's grumpy old Henry not telling Betty about Don's calls, or Megan getting frustrated at Don's obvious affection for his ex, the couple still have a hold over each other. The "everything's going to be okay" phonecall between Don and Betty will have delighted those viewers who still hold out for a reunion of the original Drapers.

When he wasn't busy worrying about "Bets" or being dragged around by Megan, Don found himself trying to snare a baked beans advertising deal with the Rolling Stones to keep team Heinz happy. The scenes with Harry Crane and Don were golden and sadly too brief. We could have watched a whole hour of Don's growing frustration at burger-scoffing Harry and we'd have not complained. And the constant deadpan knockbacks from Don whenever Harry wants to hang out with him never fail to raise a smile.

Don's disdain and fatherly concern for the screeching teen Rolling Stones fans were a marked contrast to the man we've seen in before seasons of the show. Previously, Don has always managed to hold his own or involve himself with the younger generation. Now he's become a concerned parent and an eye-roller at the swinging '60s hysteria.

As we mentioned last week, the battle of the generations remains a strong theme in Mad Men season five. Back in the offices of Sterling Cooper Draper Pryce, Pete's deal with Mohawk airlines means business is on the up, but his constant belittling of Roger is bubbling under the surface.

"When are things going to go back to normal?" moaned Roger. A simple throwaway remark perhaps, but in the hands of Roger Slattery and within the context of the episode and the series, his words seemed particularly pertinent. The company have a new black secretary, they have a new Jewish employee in Michael Ginsberg (Ben Feldman), and Pete Campbell and Peggy are the people that keep the business ticking over, not Roger. Maybe not even Don.

Roger may still have all the best wisecracks (plenty of them are offensive and at the expense of his new colleagues), but this is 1966 and things are never going to go back to what he considers "normal" ever again. How he and Don deal with that should keep us hooked for the rest of the season.

Mad Men airs on Tuesday nights at 9pm on Sky Atlantic. It airs in the US on Sundays on AMC.